1. Field of the Invention
The present invention relates to a method, a system and a program for analyzing pulse wave data used especially for extracting information concerning RR intervals, for example, from an electrocardiogram.
2. Description of the Related Art
Measurement of RR intervals on an electrocardiogram (ECG) taken from a subject is a widely used method of diagnosing cardiac arrhythmias. As shown in FIG. 19, the RR interval is a period of time between peaks of two successive R waves which are the most prominent among P, Q, R, S and T waves occurring in one heartbeat cycle on an ECG tracing. Successive readings of RR intervals, when arranged time-sequentially, make it possible to recognize heartbeat variations which are commonly used in clinical applications as an evaluation index for diagnosing biological control functions performed by an autonomic nervous system in human body activities.
Electrocardiographic measurement is performed by using a Holter monitor, for instance, which is provided with electrodes for sensing electrical potentials produced by muscular activity of the heart. The Holter monitor often used in a medical institution is a wearable device including a plurality of (e.g., five) electrodes attached to the chest of a subject and a data recorder worn by the subject at the waste or over the shoulder for recording data received from the electrodes. The Holter monitor is typically worn for 24 hours and the subject is encouraged to continue his or her daily activities. Upon completion of Holter monitoring, the data stored in the data recorder is transferred to an analyzer which performs an analysis of recorded heartbeat patterns to determine RR intervals. One chronic problem of this conventional Holter monitoring approach is that the subject is forced to endure conditions of substantial stress as the subject must continue daily activities while wearing the data recorder and the electrodes.
An alternative measurement method intended to overcome this problem is currently under study. This method is to extract information concerning RR intervals from pulse wave data without using ECGs, where a pulse wave is representative of a vasomotor response observed on the body surface as a pattern of volumetric changes of any artery caused by an inflow of blood thereinto. The pulse wave is closely related to (or synchronized with) pumping motion of the heart and, thus, it is possible to obtain information indirectly representing the RR interval by measuring movements of peripheral blood vessels.
As an example, Japanese Unexamined Patent Publication No. 1997-229013 describes a pulse wave RR interval measuring apparatus comprising a pulse wave sensor which continuously detects pulse waves of a subject and calculation means which determines peak values and peak points by analyzing pulse wave data detected by the pulse wave sensor and calculates the RR interval. Also, Japanese Patent No. 3635663 disposes an arrhythmia sensing apparatus which is configured to perform frequency analysis of a pulse waveform detected by a pulse waveform sensing device, exactly detect pulse wave components by filtering out body movement components from the pulse waveform and discover the presence of any cardiac arrhythmia using results of the pulse waveform frequency analysis. In addition, Japanese Unexamined Patent Publication No. 2001-70265 proposes a method of pulse wave analysis for determining the RR interval by calculating a first derivative of a plethysmogram (pulse wave) obtained by a pulse wave sensor to produce a velocity plethysmogram and detecting peaks of the velocity plethysmogram.
Peaks occurring in pulse wave data are however not so prominent as R waves observed on an ECG and, in addition, the pulse wave data contains small notches and reflected wave components which act as noise in a process of peak detection. These notches and reflected wave components are relatively small peaks (top peaks) and valleys (bottom peaks) in a pulse waveform. For the sake of simplification of the following discussion, the notches and reflected wave components in the pulse wave data are hereinafter referred to simply as “notches.” Due to the presence of these unwanted notches, it is not conventionally so easy to automatically detect peaks corresponding to the R waves in a raw pulse waveform. Neither the aforementioned Unexamined Patent Publication No. 1997-229013 nor U.S. Pat. No. 3,635,663 makes any mention of a method of exactly extracting peaks from the pulse wave data. Although the pulse wave analysis method proposed in Japanese Unexamined Patent Publication No. 2001-70265 is supposed to be able to remove unwanted notches to a certain degree, true peaks of the pulse wave might be judged as noise if the occurrence of such peaks is not predictable like those caused by arrhythmia. This is because an approach employed in the pulse wave analysis method of the Publication is to remove notches by using a threshold defined based on a mean value of peak intervals.